448
FXUS66 KPDT 031121
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
421 AM PDT Thu Sep 3 2026

.KEY MESSAGES...
- Periodic shower chances (35-85%) across the mountain zones
  will continue through the weekend, with lower chances in the
  lower elevations

- Isolated thunderstorm chances (15-25%) will develop across
  Cascades, Blues, and Wallowa county Today and Friday

- Below normal temperatures through early next week, with
  potential for freezing temperatures in mountain valleys
  tonight and Friday night

&&

.DISCUSSION...Water vapor satellite imagery tonight shows a
shortwave trough lifting northeast across the intermountain
PacNW, while an upper low offshore is swinging southeast towards
the coast. The shortwave trough passage this morning has
resulted in light to moderate rainfall continuing across the
eastern half of the forecast area. This activity will diminish
in the lower elevations over the next couple of hours as the
shortwave lifts into ID, with light showers continuing across
the mountains.

Today through this weekend, the PacNW will be under the
influence of the upper level low currently offshore, with
another couple rounds of rain showers in the lower elevations,
with more persistent chances across the mountains (confidence
70-90%). Temperatures across the region will be nearly 10-15
degrees below average each day, with afternoon high temperatures
at their lowest in the mid 60s and low 70s today and Friday
(confidence 80-90%). By this afternoon, the upper closed low
will park itself over the PacNW, with the center of the low
remaining west of the Cascade crest through Saturday before
sliding east Sunday into Monday(confidence 75-85%). During this
time, persistent rain shower chances (30-60%) will remain over
the Cascades and eastern mountains each day, with shortwave
impulses Friday/Saturday and the low passage Sunday/Monday
producing light rain chances (15-40%) in the lower elevations.
Isolated thunderstorms will be possible over the mountains
today and Friday as weak instability develops in these areas
with additional support from the cold core of the low aloft. As
for precipitation amounts, the Cascades and Blues will see 0.1
to 0.25 inches of daily precip (confidence 70-90%), with
locally higher amounts where thunderstorms develop.

Sunday into Monday, ensemble and deterministic members show the
upper closed low beginning to push east, with the low over the
forecast area during the day Sunday. While light to moderate
rain showers are forecast across the area, the upper low being
overhead and weak instability developing will also result in
slight chances (15-25%) of isolated thunderstorms focused across
central OR and the eastern mountains Sunday afternoon and
evening. By Monday evening, what is left of the upper level low
will have pushed east of the region, with showers diminishing
area wide.

Tuesday and Wednesday, ensemble members are in good agreement of
weak split flow developing over the PacNW as weak zonal flow
develops to the north and troughing to the south. This will
result in mostly dry conditions, light winds, and near normal
temperatures across the region (confidence 40-50%). Lawhorn/82

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...VFR conditions to
prevail through the period. Upper level low pressure system
impacting the region will produce a chance (25-35%) of light
rain showers at most sites between 22Z-06Z, except at sites
YKM/PSC where chances are much lower through the period. CIGs
may lower down to around 5kft with precipitation, with vsby
remaining above 6 miles in precipitation. Winds will be light,
12kts or less, through the period. Lawhorn/82

&&

.Preliminary Point Temps/PoPs...
PDT  68  42  68  43 /  10  40  30  10
ALW  69  46  68  47 /  20  40  30  20
PSC  73  43  73  43 /   0  10  20  10
YKM  70  45  71  47 /  10  10  20  10
HRI  71  43  71  43 /   0  20  10  20
ELN  66  40  66  41 /  30  20  40  20
RDM  64  31  63  32 /  20  40  50  60
LGD  66  41  66  39 /  80  80  60  40
GCD  63  39  65  39 /  80  70  40  30
DLS  70  47  72  48 /  30  30  20  30

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
WA...None.
OR...None.

&&

$$

DISCUSSION...82
AVIATION...82